X-ray imaging devices are generally adapted for obtaining information about internal structures within an object of interest. For example, in medical X-ray imaging, X-ray imaging devices are used to obtain information about structures within a human body. Therein, X-rays are emitted from an X-ray emitting device comprising an X-ray source and an emitted X-ray beam is transmitted through a region of interest of the patient's body. Different types of tissues or bones within the patient's body absorb or attenuate the transmitted X-rays to different degrees. The transmitted X-rays are then detected by an X-ray detector comprising a multiplicity of detector pixels or detector elements such that from a local distribution of detected X-ray intensities information about the internal structure within the patient's body may be derived.
Generally, the X-ray source emits an X-ray beam with a relatively homogeneous X-ray intensity distribution. However, upon transmissions of the X-rays through the patient's body, some portions of the X-ray beam are attenuated more strongly than other portions. For example, portions comprising bones attenuate transmitted X-rays much more than portions with soft tissue. Accordingly, an X-ray intensity detected by the detector pixels of the X-ray detector may strongly vary throughout a detection plane of the X-ray detector. Therein, some detector pixels may starve, i.e. receive only very low X-ray intensities, such that noise may become a problem. Other detector pixels may suffer from overexposure such that these detector pixels come into overload and image quality deteriorates while at the same time the corresponding parts of the patient's body obtain an excessive X-ray dose.
Generally, in a patient's body, the more X-ray absorbing parts such as e.g. the spine are arranged centrally to a region of interest whereas in peripheral regions of the body less X-ray absorption occurs.
Accordingly, so-called bow tie filters are used to smoothen a photon flux across the X-ray detector. Such bow tie filters generally have a geometry similar to a bow tie, i.e. with a smaller thickness in the center than in the peripheral regions. Typically, a bow tie filter is made from a material with qualitatively low X-ray filtration. Ideal materials would attenuate an X-ray beam without altering its spectrum. Materials with low atomic number (Z) are preferred. The photoelectric effect, which is the dominating and highly spectral sensitive physical effect of attenuation for materials with high atomic numbers, is small for these low-Z-materials. For low-Z-materials the Compton scatter effect takes over, which is a less spectral sensitive effect of attenuation in the range of photon energies, which are relevant for human CT. For example, PTFE (Teflon®) may be used for the bow tie filter. However, as the X-ray attenuation per unit length is low, as the photoelectric effect is reduced, a bow tie filter typically needs to have a relatively large thickness of up to several cm. Accordingly, in an X-ray imaging arrangement such as in a CT (computer tomography) scanner, the bow tie filter occupies valuable space. Furthermore, beam hardening, i.e. a shift of the transmitted X-ray spectrum towards a higher average X-ray energy, cannot be completely avoided upon use of a bow tie filter, even if it is made from a low-Z-material. Both these properties may be an issue for example for helical scanning and spectral sensitive CT.